The rough-in is measured from the finished wall.
Standard is 12" from finished wall to center of drain.
The water supply can be 8" to the left of center, about 6" high.
Most walls finish off with 1/2" drywall, however some use layer of tile or wainscoting over the drywall too.
The side measurements should be 15" to center, therefore you would need at least a 30" space to put a toilet in.
You should have at least 24" in front of the bowl too.